The reduction-oxidation (redox) flow battery is an electrochemical storage device that stores energy in a chemical form and converts the stored chemical energy to an electrical form via spontaneous reverse redox reactions. The reaction in a flow battery is reversible, so conversely, the dispensed chemical energy can be restored by the application of an electrical current inducing the reversed redox reactions. Hybrid flow batteries are distinguished by the deposit of one or more of the electro-active materials as a solid layer on an electrode. Hybrid batteries may, for instance, include a chemical that forms a solid precipitate plate on a substrate at some point throughout the charge reaction and may be dissolved by the electrolyte throughout discharge. During charge, the chemical may solidify on the surface of a substrate forming a plate near the electrode surface. Regularly, this solidified compound is metallic. In hybrid battery systems, the energy stored by the redox battery may be limited by the amount of metal plated during charge and may accordingly be determined by the efficiency of the plating system as well as the available volume and surface area to plate.
One example of a hybrid redox flow battery is an all-iron redox flow battery (IFB), which uses iron as an electrolyte for reactions wherein on the negative electrode, Fe2+ receives two electrons and deposits as iron metal during charge and iron metal loses two electrons and re-dissolves as Fe2+ during discharge, as shown in equation (1). On the positive electrode, two Fe2+ lose two electrons to form Fe3+ during charge and during discharge two Fe3+ gains two electrons to form Fe2+, as shown in equation (2):Fe2++2e−↔Fe0 (Negative Electrode)  (1)2Fe2+↔2Fe3++2e− (Positive Electrode)  (2)
On the negative electrode of an IFB, the ferrous iron reduction reaction competes with two side reactions: the reduction of hydrogen protons H+ (reaction (3)), wherein two hydrogen protons each accept a single electron to form hydrogen gas, H2, and the corrosion of deposited iron metal to produce ferrous ion Fe2+ (reaction (4)), respectively:H++e−↔½H2 (Hydrogen proton reduction)  (3)Fe0+2H+↔Fe2++H2 (Iron corrosion)  (4)
These two side reactions reduce the overall battery efficiency, because electrons transferred to the negative electrode are consumed by hydrogen production rather than by iron plating. Furthermore, these side reactions result in imbalanced electrolytes, which may cause battery capacity loss over time.
To minimize these side reactions, it is preferable to maintain the negative electrolyte of an IFB within a pH range of 3 and 4, where the ferrous ion (Fe2+) in the negative electrolyte remains stable and the rates of reactions (3) and (4) are slow. In the positive electrolyte, however, ferric ion (Fe3+) is only stable at pH less than 2. Indeed, to minimize ferric hydroxide, which is non-conductive and hinders reaction (2), and further to promote redox reaction kinetics, a pH value of around 1 is desired for the positive electrolyte.
Ionic movements of H+ and Fe3+ across the membrane barrier separating the electrolytes can be deleterious to the performance of an IFB battery. These ionic movements are driven by diffusion, migration and convection. As H+ crosses from the positive electrolyte to the negative electrolyte during battery charge, the pH of the positive electrolyte rises. When the pH of the positive electrolyte is 2 or above, Fe3+ in the positive electrolyte precipitates as Fe(OH)3. Furthermore, when Fe3+ crosses over from the positive electrolyte (more acidic) to the negative electrolyte (less acidic), Fe(OH)3 can also form in the negative electrode and/or on the membrane separator. This Fe(OH)3 formation is the root cause of electrolyte instability and poor cycle performance of an IFB battery, because the Fe(OH)3 precipitate can increase membrane separator resistance by fouling the organic functional group of an ion exchange membrane or clogging the small pores of the micro-porous membrane. Further, the Fe(OH)3 precipitate is non-conductive, so once it precipitates on electrode surfaces, it degrades electrode performance.
Thus, long term performance stability of an IFB battery may be increased by eliminating Fe(OH)3 precipitation formation. The formation of the Fe(OH)3 precipitate on the positive side may be eliminated by maintaining the pH of the positive electrolyte around 1 and the formation of the Fe(OH)3 precipitate on the negative side may be eliminated by reducing crossed-over Fe3+ to Fe2+, which is stable in a pH range from 3 to 4. To accomplish both objectives, an electrochemical cell may be implemented, wherein hydrogen gas evolved from the IFB battery negative electrode (reaction (3) and (4)) flows through the electrochemical cell anode and the positive/negative electrolytes of the IFB battery flow through the electrochemical cell cathode. The electrochemical cell anode and cathode may be electrically connected, such that the electrochemical reactions occurring at the anode and cathode of the electrochemical cell convert gaseous hydrogen back to protons in order to maintain electrolyte pH and consume the crossed-over Fe3+ to Fe2+, which may thus result in clean and stable IFB electrolytes.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.